1. Related Application
The Assignee of the present invention is also the owner of design patent application Ser. No. 602,377, filed Apr. 20, 1984, for PASSIVE GEOPHYSICAL EXPLORATION DEVICE.
2. Field of the Invention
The present invention relates to geophysical prospecting involving the passive analysis of the earth's telluric currents and, in particular, to a portable analog apparatus and method therefor.
3. Discussion of the Prior Art
It is well known that the earth has naturally occurring sheets of telluric currents which flow along the earth's surface. Dobrin, "Introduction to Geophysical Prospecting", McGraw Hill (3rd Ed. 1976) pg. 591-619. The generation of these telluric currents are believed to be induced just below the earth's surface by the ionosphere, due to the solar influence on the atmosphere, by lightning discharges and by storms such as tornadoes. Id. at 596 and Wait, "Geo-Electromagnetism" (1982) Academic Press Inc., pg. 184. According to Wait, those telluric currents having durations of less than one or two seconds, with frequencies particularly in the audio portion of the radio spectrum, are produced primarily by lightning discharges.
Geophysical prospecting involving only the passive sensing of telluric currents has been reported by Dobrin, to have occurred as early as 1939 by Marcel Schlumberger and has found wide application for oil exploration primarily in Europe, North Africa, and Russia. Dobrin at 591. Telluric geophysical prospecting is passive and is unlike other forms of electromagnetic prospecting which require externally or artificially induced currents.
A number of passive telluric prospecting methods are set forth by Dobrin, Id., to include the telluric current method, the magnetotelluric method, and the natural alternating magnetic field method. An example of the latter is the AFMAG (Audio Frequency Magnetic Field) method for measurement of the natural oscillating magnetic fields at audio and sub-audio frequencies. The pulsating electromagnetic fields from the nonstatic and time variable telluric currents have an electric field, E(t), and a corresponding magnetic field, H(t), wherein: EQU E(t)=ZH(t) Formula 1
The complex impedance of the earth, Z, depends upon the magnetic, dielectric and conductivity properties of the earth. The electric field components are almost vertical and the magnetic field components are almost horizontal. The AFMAG method detects the horizontal magnetic components by means of coils or loops. By detecting two separate frequencies, such as 150 and 510 Hz, the ratio of the responses detected provides a measure of the conductivity of the underlying strata. According to Dobrin, the AFMAG method was particularly suitable for the detection of deep-seated structures such as faults and buried vertical dikes. Dobrin at 601. AMT (or Audio Magneto-Tellurics) utilizes frequencies from 20 to 4000 Hz, while MT (Magneto-Tellurics) uses frequencies from 1/1000 to 1 Hz. AMT uses normally a set of two or three fixed frequencies, e.g., 100, 200, 500 Hz and determines impedance, Z.
The magnetotelluric method plots the ratio of the earth's alternating magnetic field to the alternating electric field as a function of frequency to provide information on the variation of resistivity with depth. Hence, magnetotelluric methods involve the simultaneous measurement of the E(t) and H(t) fields at the same location over a frequency range. The method records five parameters from five detectors which sense both horizontal components of the E field and H field as well as the vertical component of the H field. The E fields are typically measured by nonpolarizing porous-pot electrodes made up of cadmium immersed in a cadmium chloride solution. Dobrin at 596.
Under the telluric current metod, electrodes are properly located on the ground to sense the voltage difference between locations caused by the oscillatory telluric currents. Dobrin at 592. As will be subsequently discussed the present invention is not properly categorized in any one of the above three categories.
It is recognized that a low frequency window (LFW) exists when telluric currents pass through the earth's substrata. In the frequencies of the LFW, the earth acts as a conductor. Burrell et al., "Pulse Propogation in Lossy Media Using the Low Frequency Window for Video Pulsed Radar Application", Proceedings of the IEEE, Vol. 67, No. 7, July 1979, pgs. 981-990. In the low frequency window, it is known that electric field waves coming from below the surface, upon impacting the interface between earth and air, approximately doubles because the voltage reflection coefficient is approximately plus one whereas the magnetic field, having a reflection coefficient of minus one, cancels. Id. at 982. Hence, it is recognized that antennas lying on the surface of the earth respond to such electric signals to generate a response. Id.
The low frequency window, LFW, has been recognized to exist from zero up to a cutoff frequency, f.sub.c, as follows: ##EQU1## where: 2d=the distance to the observance point in meters
.sigma.=the conductivity of the medium in mhos/m PA1 f.sub.c =the frequency at which the electric amplitude is 3 dB less than the value at zero frequency.
Id. at 984. Generally, the low frequency window is in the audio range and extends from zero Hz to several kHz depending on the earth's conductivity and depth.
In the above frequency cutoff formula, the determination of f.sub.c involves two unknowns (d and .sigma.). The present invention provides a means to determine f.sub.c through a single observation of the electric field, E(t), of the telluric current. By definition depth, d, is a continuous variable, strongly influencing (by the square) the cutoff frequency, f.sub.c ; whereas the conductivity, .sigma., is a piece-wise constant variable changing only with a change in lithology (with each different strata layer) by less than a power of magnitude.
In FIG. 1a, the relationship between the log of the cutoff frequency and the log of the depth is shown for different values of conductivity. For example, for a conductivity of 10.sup.-2 mhos/m, as the depth increases to four units, the cutoff frequency drops correspondingly. It can be seen that at a depth of three units, the cutoff frequencies for different conductivity values stepwise changes. Therefore, if different substrata layers have different values of conductivity, the cutoff frequency is affected in a piece-wise constant manner. The resistivities of various typical substrata materials are set forth in the American Institute of Physics Handbook McGraw-Hill (1957), pg. 5-290, FIG. 5k-2. For example, depending on mineralization, sea water or brine has a resistivity of less than one ohm-meter whereas anhydrite has a resistivity of greater than 10,000 ohm-meters.
The present invention utilizes these distinctions between the depth, d, (i.e., affecting the cut-off frequency by the square) and the conductivity, .sigma., (i.e., affecting the cutoff frequency in a piece-wise or step function) to provide an indication of both the depth and the nature of substrata.
The present invention also takes advantage of another characteristic of telluric currents. The field pulsations originating from ionospheric and atmospheric sources also induce, in hydrocarbon or mineral deposits as shown in FIG. 1b, at a given depth, d, at a different conductivity, .sigma..sub.2, a secondary telluric current, I(t), flowing at the boundaries of the volume, V, of the deposit in the form of a dipole moment I(t)L given by the following equation: EQU I(t)L=(.sigma..sub.1 -.sigma..sub.2)E(t)V Formula 3
where E(t) is the primary electric field strength penetrating the earth from the surface through the low frequency window to that depth. This is discussed in Cauterman, et al. "Numerical Modeling for Electromagnetic Remote Sensing of Inhomogeneities in the Ground", Proceedings of the IEEE, Vol. 67, No. 7, July, 1979, pg. 1010. The dipole moment I(t)L consists of a dipole distribution at the borders of the deposit, which produces a secondary pulsating electromagnetic field. These secondary field pulsations are delivered towards the surface of the earth in the form of an upgoing series of audio pulses, also band limited by the LFW. When these secondary pulses reach the surface, the following effects take place.
First, the electric field on the surface, as mentioned, is doubled whereas the magnetic field on the surface cancels. Secondly, the electromagnetic radiation reaching the surface from the air above behaves the opposite way. In other words, the electric field cancels on the surface because the reflection coefficient of the atmospheric electric field is minus one whereas the atmospheric magnetic field doubles.
Hence, the present invention is designed to sense the primary telluric currents in order to provide an indication of both the depth and nature of the substrata beneath the earth's surface and is further designed to sense the secondary telluric currents to provide an indication of the presence of hydrocarbons, minerals, and other inhomogeneities in the ground.
In FIG. 2 is set forth another type of prior art passive telluric current detector which utilizes a coil 200 oriented vertically to ground capable of detecting magnetic field component. The core of the coil is soft iron. A tuner 230 is connected to winding 210 over leads 212 and essentially comprises a variable capacitor which provides the tuning to the coil 200. The output of the tuner 230 is delivered into several stages of amplification 240 over leads 232. Part of the signal at the output of the amplifier 240 is delivered over line 242 to a second winding 220 which is wound opposite to winding 210 for approximately one-third the length of the coil 200. The output of the amplifier 240 is delivered over lines 242 to a detector 250. The detector is then interconnected over lines 252 to a pair of headphones 260. The details of the schematic shown in FIG. 2 are only partially known to the inventors and it is not understood by the inventors how the instrument truly operates. However, it is believed that the instrument detects magnetic waves H(t) with the coil 200 and delivers a signal over leads 212 into a tuner 230. The signals on leads 212 charge the tuning capacitor 230 and that signal is delivered into the amplifier 240 where a feed-back over line 240 effects a 180 degree phase shift to the signal causing the capacitor in tuner 230 to discharge thus enabling the tuner 230 to fully track the upcoming magnetic pulses which vary in time. It is believed that the phase shift caused by the feed-back on line 242 results in a composite signal on 212 that is comprised essentially of residues marking the beginning and the end of each charging pulse. The detector 250 of FIG. 2 is believed to be essentially a spike limiting amplifier which is necessary to eliminate man-made inference. The resulting pulse residuals are delivered over leads 242 to the detector 250 and then monaural audio signals are delivered into earphone 260. The prior art approach of FIG. 2 has been observed to work occasionally and only with a highly trained operator. The prior art approach works best during active or magnetic storm periods or for the detection of shallow highly conductive mineral veins.
While the aforesaid technique appears to directly measure the magnetic field, the measurement of the corresponding electric field in the atmosphere is discussed in Estermann, "Classical Methods", Academic Press (1959), pgs. 502-503. One early approach for measuring the electric field discussed in Estermann is the 1905 Wilson test plate method which utilized a horizontal conducting plate, a few square centimeters in area, connected to an electrometer. A shield is placed over the test plate. The Wilson technique provides an apparatus for measuring the earth's electric field by sensing the charge buildup on the test plate. Like the prior art approach discussed above in FIG. 2, the Wilson approach utilizes a variable capacitor to discharge the plate. The use of an electrometer or galvanoscope for sensing magnetic field or electric fields are well known. See Kupfmuller, Einfuhrung In Die Theoretische Elekrotechnik, "Introduction to Theoretical Electricity", Springer, Berlin (1932) pg. 143. However, these sensors are believed not to have been applied to geophysical prospecting systems.
The present invention, contrary to the above prior art, is a passive analog electromagnetic indicator responsive, in the preferred embodiment, to the electric field E(t), of the naturally occurring primary and secondary telluric currents in the earth through the low frequency window to depths of many thousands of feet in order to determine the presence of inhomogeneities such as oil, gas, water, coal, and minerals and additionally to provide an indication of their depth from the surface including a determination of the lithology of the overburden (i.e., limestone, sandstone, shale, and sands) and to specify the top and bottom depth of such overburdens. The present invention, in the preferred embodiment, is a field portable, battery operated, instrument which operates on or slightly above the earth's surface without the necessity of drilling a hole and which provides a stereo output to enhance the recognition of the telluric information.
In contrast to the above discussed prior art approaches especially that of FIG. 2 and the Wilson test plate method, the present invention matches the impedance of the sensor to the impedance of the ground in order to fully couple to the upcoming telluric current. In this fashion, the antenna in the horizontal position becomes part of the ground. In addition, the indicator of the present invention utilizes a low ohmic resistance to discharge the charge buildup in the sensor in synchronization with the electromagnetic field. The operator of the present invention is provided with a state variable filter and a deconvolution technique for indicating a solution to Formula 2 and, finally, by utilizing a stereo audio output, compensated for the operator's ear sensitivity and background noise, the signal interpretation produced by the indicator is significantly enhanced.